Several kinds of measurements are carried out today in the process industry. These measurements require the direct or indirect connection of the measuring instrument such as the sensor, pressure mediator or the like with the process substance. The measuring instrument can be brought into contact with the process substance in several ways. The most simple way for mounting for example a sensor in its place is to attach by welding the sleevelike body of the connecting device to the boundary surface of the process, for example to the outer surface of a tube. The sleevelike body is most suitably provided with an internal threading or a fixing flange and the boundary surface of the process is provided with an aperture at the body. The sensor is fitted through the sleevelike body into contact with the process substance and it is secured thereto for example by means of external threading. The sensor is thus directly connected with the process. Other connecting devices known in the art include devices, which are connected with the process so that a shut-off valve is disposed between the body of the connecting device and the process. By means of this valve the contact between the process substance and the connecting device can be shut off when necessary. This kind of an arrangment for connecting a device for measuring instruments is disclosed in CH-patent publication No. 521,657. The sensor in the connecting device is movable in its longitudinal direction. To start the measurement the shut-off valve must first be opened and the sensor is then moved in its longitudinal direction through the hole of the shut-off valve to its front position into contact with the process. In a corresponding manner, when finishing the measurement the sensor must be moved in its longitudinal direction out of the hole of the valve to its back position and the valve is then closed.
A connecting device which is connected directly with the process and having a sensor attached to its body, which is in turn fixed to the boundary surface of the process has a disadvantage, in that replacement or maintenance of the measuring instrument cannot be carried out without disturbing the process itself. Before the actual replacement or maintenance work of the sensor can be started, the process must be interrupted at least in the vicinity of the connecting device. The connecting device equipped with a shut-off valve has a disadvantage, that a relatively large number of assembly parts are required. First, a fixing member attached to the boundary surface of the process is required. The valve is affixed to this fixing member. The actual connecting device, which engages the sensor is then attached to the valve. The use of this measuring arrangement is laborious, because the sensor and the shut-off valve must be moved separately to accomplishing the connection between the sensor and the process substance or to cut off the connection. Moreover, because the shut-off valve must always be located at a relatively long distance from the process, the connecting duct between the process substance and the shut-off valve can become clogged, if the process substance to be measured easily forms precipitates. In any case, the sensor must displace the process substance entering into the connecting channel and eventually also into the shut-off valve. This may involve great difficulties, when the sensor is connected with the process substance after the shut-off period, and in particular when high-pressure processes are measured.